1. Field of the Invention
The invention in general relates to pyroelectric sensors, and in particular such a sensor that utilizes a ferroelectric sensing element.
2. Statement of the Problem
Pyroelectric sensors that utilize ferroelectric sensing elements are known in the art, such as described in U.S. Pat. No. 6,339,221, which is hereby incorporated by reference as though fully disclosed herein. An electrical schematic of an active pyroelectric sensor 100 as disclosed in U.S. Pat. No. 6,339,221 is shown in FIG. 1. Sensor circuit 100 includes a source 102 of a voltage pulse Vs, a ferroelectric capacitor 104, a linear storage capacitor 108, an operational amplifier 110, and diodes 112 and 114. Electric pulse source 102 is connected between ground 109 and one electrode of ferroelectric capacitor 104. The other electrode of ferroelectric capacitor 104 is connected to the cathode of diode 114 and the anode of diode 112. The anode of diode 114 is connected to ground 116 and the cathode of diode 112 is connected to the inverting input of operational amplifier 110 and one electrode of capacitor 108. The other electrode of capacitor 108 is connected to the output 120 of op amp 110. The non-inverting input of op amp 110 is connected to ground 118. Connected in this form, capacitor 108 and operational amplifier 110 form an integrator 115. Sensor 100 operates as follows. An AC signal Vs is applied to the ferroelectric capacitor 112 to switch the polarization which causes a polarization current, If, to flow to integrator 115 which integrates the pulses of ferroelectric switching current. Diodes 112 and 114 filter the switching current, allowing only the positive part of the pulse to flow to integrator 115, while the negative portion flows to ground 116. The ferroelectric polarization of capacitor 104 changes with temperature, which is known as the pyroelectric effect. The output voltage, V0, of operational amplifier 110 will depend on the polarization of capacitor 112, and thus on the temperature. The change in polarization of capacitor 104 with temperature is small; however, the integration of many pulses by integrator 115 permits the temperature change to be sensed as indicated by a change in the output voltage V0.
Analyzing the design of sensor 115, if C0, the capacitance of capacitor 108 is equal to Cf, the capacitance of ferroelectric capacitor 104, V0, will reach its maximum after 1 cycle. Therefore, if multiple pulse cycles are to be integrated, C0 must be greater than nCf, where n is the number of pulse cycles integrated, otherwise V0 will saturate. Assuming C0=nCf, V0 will gradually increase with the number of applied cycles, and reach its maximum. The maximum of V0 is:nPr/C0=Pr/Cf,  (1)where Pr is the remnant polarization of the ferroelectric capacitor 112. If there is a temperature change, the maxim change of V0 is:ΔV0=ΔPr/Cf=AfpyΔT/Cf,  (2)where Af is the area of the ferroelectric capacitor and py is the pyroelectric coefficient. Since the change of V0 does not depend on n, an increase of n will not increase the maximum of V0, or the maximum change of V0. Thus, the ability of sensor 100 to sense changes of temperature depend on the area and capacitance of the ferroelectric capacitor, which creates constraints on the design of the circuit, which limits the ability of the circuit 100 to sense small changes of temperature.
It would, therefore, be highly desirable to have a ferroelectric pyroelectric sensor in which simply by increasing the number of integration cycles the sensitivity of the sensor could be increased.